Streaming current detectors take advantage of physical phenomena in which an ionic flux is produced by rapid movement between a pair of spaced apart, but close-fitting dielectric elements, the surfaces of which carry electrical charges induced by a charge-bearing fluid which bathes both elements. Ions or charged colloids adsorbed on the dielectric wallsxe2x80x94which may include remnants of an earlier test fluidxe2x80x94give this ionic flux its unique characteristics. In a typical streaming current detector, the spaced apart dielectric elements are fabricated of Teflon R and include a piston and a bore for slideably receiving the piston; clearances between them measure about 0.005 inch.
To transmit a xe2x80x9cstreaming currentxe2x80x9d, two electrodes, interconnected by an external circuit, are mounted proximate with the paired dielectric elements. A popular combination, taught by Bryant and Veal in U.S. Pat. No. 4,769,608, is to secure ring-like electrodes to the wall of the bore. An electrical signal is generated when the fluid attains a state of hydrodynamic shear as the close-fitting dielectric surfaces move past each other. The strength of this signal is dependent upon, among other things, the conductivity of the fluid, its velocity, the size of the fluid passageway, and the frequency of oscillation of the piston, as well as the presence of adsorbed species on the piston and bore surfaces.
While the usefulness of streaming current measurements for controlling the amount of chemicals needed to treat water, as well as various wastewater streams, is now widely recognized, operators trying to obtain such measurements, and have them be both continuous and reliable, still face daunting challenges. Not only does the buildup of contaminants on the electrodes or surfaces disposed proximate with them degrade the signal but also its strength can be reduced, with detrimental consequences, by numerous factors. Among them is an increase in the conductivity of the test fluid, a change which by itself can dramatically attenuate the streaming current signal. Indeed, if the conductivity goes high enoughxe2x80x94to at least about 10,000 micromhos, the electrodes themselves short out. An operator could think, based on his streaming current observations, that he needs to add fewer chemicals or replace the dielectric elements and/or electrodes, when the real problem is that the process flow stream has experienced a sudden, unexpected increase in its electrolyte concentration.
Nor has a long-standing debate as to what the streaming currentxe2x80x94sometimes referred to as the xe2x80x9cstreaming potentialxe2x80x9dxe2x80x94actually represents helped this situation. Getting beyond the basic assumption, i.e., the streaming current is related in some way to the surface charge or surface potential (zeta potential) of dispersed colloidal particles in a given system,.to an understanding of the zeta potential-streaming current detector response interrelationship remains an elusive goal. In view of this uncertainty, no one ventured to equip any of the prior art detectors with a compensating conductivity probe, even though the pronounced effects which changes in conductivity have on the streaming current are well known.
To circumvent conductivity-related distortions of the streaming current signal, prior art detectors have been relegated to a minor role in applications where the electrolyte concentration in a process flow stream varies widely. As part of an elaborate titration apparatus, the detector is used only to indicate when, as each discrete batch of test fluid is being titrated, the streaming current vanishes. The complexity of this apparatus introduces its own set of technical problems, as Krah discloses in U.S. Pat. No. 5,408,185.
On the other hand, progress towards eliminating streaming current aberrations caused by slow fouling is apparent in the prior art. Recognition came rather quickly that desorbing high molecular weight polymers once they are adsorbed on a Teflon R or similar dielectric material is virtually impossible in an on-line instrument. Instead, Bryant,and Veal invented an electrode holder which can be removed and replaced in less than one minute. As disclosed in U.S. Pat. No. 5,119,029, this holder included both the bore and the electrodes in a single unit. Problems with its use arose when operators seeking to rehabilitate holders soaked them in cleaning solution and inadvertently shorted out the electrical connections to the electrodes.
Attempts to provide reliable streaming current signals in situations where fouling occurs abruptly have been less successful. Problems with scaling in certain situations are so severe as to make the use of prior art detectors, including those equipped with removable electrode holders, cost prohibitive. In particular, lime scalingxe2x80x94which occurs wherever lime, a popular water treatment chemical, is used to coagulate/precipitate contaminants from a flow streamxe2x80x94has been inherently difficult to deal with.
Other conditions under which prior art remedies have proven inadequate involve the presence of grit or heavy silt in the test fluid. Not only do grit and heavy silt scratch less wear-resistant dielectric surfaces but also they introduce hairline crevasses along which charge-laden debris can accumulate. Jammed into the narrow fluid flow channels in a streaming current detector, grit can even cause a bore-mounted ring electrode to break or become dislodged. Pre-sample filters, strainers and hydrocyclone samplers have all been introduced to reduce grit, heavy silt and debris build-up within streaming current detectors but have failed to eliminate these troublesome impediments. dislodged. Pre-sample filters, strainers and hydrocyclone samplers have all been introduced to reduce grit, heavy silt and debris build-up within streaming current detectors but have failed to eliminate these troublesome impediments.
An object of the present invention is to provide a streaming current detector in which the replacement of critical parts used to generate the test signal, such as the dielectric elements and the electrodes, can be accomplished quickly, easily and independently of each other, so that the detector can be used in situations in which the corrosive and/or abrasive nature of the test fluid formerly made any use of a streaming current detector cost prohibitive.
A further object of the present invention is to provide such a detector in which the dielectric elements and the electrodes can be replaced in the field in less than one minute, virtually eliminating downtime.
A still further object of the present invention is to provide an improved streaming current detector having a matched set of removable dielectric elements, respective matched sets being designed in the basis of their capacity to withstand abrasive environments, as well as their chemical compatibility with the test fluid and the operating temperature of the process flow stream.
A still further object of the present invention is to provide such a detector in which the matched set of dielectric elements can be selected to increase the strength of the test signal over that generated by a conventional piston and bore combination, a tight-fitting pair being used in applications where extra response is needed. Alternately, a xe2x80x9cloose-fittingxe2x80x9d pair can be selected for those situations in which large particles are present or the sensitivity needs to be reduced.
The intent of the present invention is also to provide for electrodes that are less vulnerable to breakage during ruse and can be easily inspected and removed for cleaning, so that the remaining components of the detector can be soaked in solution without simultaneously shorting out electrode connections.
Another object of this invention is to monitor the conductivity and streaming current of a test fluid simultaneously, so that sources of test signal variations can be better understood, especially in common applications such as municipal wastewater treatment where wide swings in conductivity often occur as a result of road salting, an event which has delayed, largely unpredictable impacts as melting runoff enters a sewerage collection system.
Accordingly, there is provided a very stable: and reliable detector for the measurement of the streaming current in water or wastewater that can be operated over long periods of time, nearly continuously, without having to be shut down for either cleaning or repair/replacement of critical parts. The detector includes a probe with a housing, an elongated hollow sleeve for slideably receiving a reciprocating piston, and the piston itself. Sensing electrodes, which are positioned proximate with the distal ends of the sleeve, are mounted on immobile structures other than the sleeve. These structures include the housing and a retaining fitting.
Prior to use, the sleeve is slideably inserted into a cylindrical void formed in the housing and secured therewithin by the retaining fitting. A shoulder on the housing forms a stop which abuts the upper end of the sleeve when it is inserted as far as possible into the housing. The retaining fitting, in the preferred embodiment, is a closure plug threadedly engageable with internal threads formed within the lower end of the housing. In the assembled detector, as the retaining fitting is tightened on these threads, a gasket juxtaposed between it and the sleeve is compressed, forming a leak-tight seal between the fitting and the housing.
The housing further defines a transverse passageway fluidly connected to the sleeve. As the piston reciprocates, samples of a test flow stream directed through the transverse passageway, are alternately sucked into and expelled from a narrow, elongated flow channel of capillary width formed between the piston and the sleeve. The test flow stream flowing in the transverse passageway moves over the flow channel entrance with sufficient velocity to wash away any floc that might otherwise accumulate there.
Like the sleeve, that portion of the reciprocating piston which it slideably receives, i.e., the xe2x80x9cactive segmentxe2x80x9d, can be easily replaced within the improved detector. Distal from the active segment, the piston is threadedly engaged with a guide, the only direct linkage between the piston and a mechanism for forcing it into repetitive upward and downward motions. Access to the piston above its active segment requires removal of the housing itself, a feat which, in the preferred embodiment, can be accomplished by disengaging a single pinch bolt or, alternately, a thumbscrew. This fastener is used to secure the housing to a downwardly protruding structural member through which the guide slides; and the structural member in turn is affixed to a protective casing for a synchronous motor and other components of the piston-driving mechanism.
In addition, each of the sensing electrodes is readily accessible and, like the sleeve and the reciprocating piston, can be removed independently of other probe elements. Generally, removal and replacement of each probe element can be accomplished in the field in less than one minute. No special tools are required.
In the preferred embodiment, the sensing electrode mounted on the closure plug can also be unscrewed with this plug, giving a user the option of simultaneously replacing both it and the sensing electrode or discarding only the electrode and installing a new one in its place.
Each electrode, whether it is mounted on the retaining fitting or the housing itself, can be threadedly advanced into the probe, so that the strength of the streaming current signal, diminished as electrode surfaces wear down, can be regained. Moreover, the electrodes in the improved detector are mounted so that they contact the test fluid just outside of the region where the close-fitting dielectric surfaces slide past each other. Thus, the rate at which the electrodes erode is reduced relative to what it would be if they were mounted inside these regions, as is the likelihood of the electrodes breaking or becoming dislodged.
Since the cost to make the dielectric sleeve is about {fraction (1/100)}th the cost of the removable electrode holder taught by Bryant and Veal in U.S. Pat. No. 5,119,029, consumers can afford to use the improved detector under conditions in which the corrosive and/or abrasive nature of the test fluid formerly made use of a streaming current detector cost prohibitive. The detector can even be used in situations in which there is severe lime scaling with this advance, a vast array of wastewater treatment processes can be controlled on-line, for the first time, using streaming current detectors.
Generally, the substantially lower cost of the sleeve makes more frequent replacement of both dielectric elements practicable. The more often they can be replaced, the more often the probe can be restored to its xe2x80x9coriginalxe2x80x9d condition, thereby obviating the test signal-degrading effects of foulingxe2x80x94whether due to scaling, chemical adsorption, or oil depositionxe2x80x94and of wear which alters the flow channel between the sleeve and-the active segment. Moreover, utilizing a new set of an otherwise identical sleeve and piston combination allows an operator to precisely calibrate the response of a system when a different chemical additive is introduced into a process flow stream; remnants of an earlier additive adsorbed on the dielectric elements can no longer distort the results.
In the preferred embodiment, the sleeve and piston are a matched set fabricated from materials and having clearances that are optimized for use with a particular test fluid. Preferably, sleeve/piston sets formulated of scratch-resistant ceramics are employed when abrasives are present in the sample flow stream and of glass when exposure to highly corrosive substances is likely. Moreover, as a rule, loose-fitting sleeve and piston combinations work best when grit and silt or other large particles are present in the test fluid.
Removable matched piston and sleeve sets also introduce a way for operators to adjust the strength of the streaming current signal mechanically. Heretofore, such adjustments were limited to manipulating electronic components. By using a xe2x80x9ctight-fittingxe2x80x9d matched set, one in which there is less clearance between the sleeve and the active segment, an extra strong signal response can be obtained. Alternately, the sensitivity of the detector can be reduced by utilizing a xe2x80x9cloose-fittingxe2x80x9d sleeve/piston pair.
Together with a first pair of sensing electrodes coupled to an electronic circuit which detects an alternating current flowing between them, the improved detector further comprises a second pair of sensing electrodes. The latter, which is preferably mounted on the housing in close proximity both to one of the first pair of electrodes and to the transverse passageway, is part of a conductivity probe integrated into the improved detector. A feed forward signal, proportional to a direct current flowing between the second pair of electrodes, is used to increase or decrease the amplifier gain for the streaming current raw signal in such a way that the latter is modified in direct proportion to changes in the direct current. The net result is that a conductivity-compensated streaming current is generated. Alternately, the conductivity can be measured independently of the streaming current.